Calculating the number of days between two dates in JavaScript (the right way)

"How many days until the deadline?" sounds trivial until you actually write the code and get bitten by time zones, daylight saving time, and off-by-one errors. Here's a clear, correct way to do it.

The naive version (and why it's slightly wrong)

The classic one-liner subtracts two Date objects and divides by the number of milliseconds in a day:

const msPerDay = 1000 * 60 * 60 * 24;

function daysBetween(a, b) {
  return Math.round((b - a) / msPerDay);
}

daysBetween(new Date("2026-01-01"), new Date("2026-03-01")); // 59

This works most of the time, but it silently breaks across a daylight-saving boundary, where a "day" is 23 or 25 hours long. Math.round hides the error for short ranges and produces off-by-one results for longer ones.

The correct version: normalize to UTC midnight

Strip the time component by converting both dates to UTC midnight before subtracting. Now every day is exactly 24 hours:

function daysBetween(a, b) {
  const utcA = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
  const utcB = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate());
  const msPerDay = 1000 * 60 * 60 * 24;
  return Math.round((utcB - utcA) / msPerDay);
}

Because we've zeroed out hours, minutes, and seconds, DST no longer matters — the subtraction is always a whole number of days.

Inclusive vs. exclusive counting

One question trips people up: does the count include both endpoints? "From the 1st to the 3rd" is 2 days if you count gaps, but 3 days if you count calendar days touched. Decide which your feature needs and add + 1 for the inclusive version. Being explicit about this in your UI saves a lot of support tickets.

Don't reinvent it every time

For a quick answer without writing code, I use CalcPine's days between dates calculator — it handles the inclusive/exclusive distinction and shows the breakdown in years, months, and days, which is handy for sanity-checking your own implementation.

Reaching for a library?

If you're already pulling in a date library, date-fns has differenceInCalendarDays(b, a) and Luxon has DateTime.diff. Both normalize calendar days for you. But for a single count, the UTC-midnight trick above is a dependency-free four-liner worth keeping in your snippets folder.

Happy shipping.